Gas furnaces are widely installed in homes for heating purposes. One common type of furnace is an induced-draft gas furnace. In an induced-draft gas furnace, a gas valve typically establishes the flow of gas into a combustion chamber while a motor-controlled blower induces air and combustion gases through the combustion chamber. In order to improve the operating efficiency of induced-draft gas furnaces, attempts have been made in developing variable draft-induced gas furnaces. Generally, the two types of variable draft-induced gas furnaces are known as multi-stage systems and modulating systems. When the user of a multi-stage system selects a thermostat setting, the system signals the gas valve to supply gas to the combustion chamber at a fixed rate corresponding to the selected thermostat setting. The system also signals the blower motor to induce a draft through the combustion chamber at a fixed rate corresponding to the gas flow rate.
Modulating systems typically utilize variable-speed blower motors and electronically modulating gas valves. Modulating systems vary the gas valve outlet pressure by varying an electronic signal to the gas valve. Thus, a modulating system can provide more precise control over gas flow than possible in a conventional multi-stage system. Attempts have been made in designing a modulating gas valve by the Carrier Corporation, the assignee of the present disclosure. Carrier Corporation disclosed in U.S. Pat. No. 5,860,411 a modulating gas valve that is responsive to pressure changes in order to regulate gas flow with the flow level of combustion air. In another U.S. Pat. No. 6,321,744, the Carrier Corporation disclosed a throttling valve responsive to differential pressure signals and furnace control signals, as well as inducer and blower motors responsive to speed control signals, in order to improve fuel utilization efficiency. Although modulating gas valves and variable-speed motors have been disclosed for modulating systems, a need for a thermostat algorithm for modulating systems still remains in order to gain greater control of the modulating system and optimize operational efficiency.
Another common feature used with gas furnaces is a two-stage thermostat to control the furnace. The two-stage thermostat allows dual rate heating, wherein the first stage of the thermostat operates the furnace on low heat and the second stage of the thermostat operates the furnace on high heat. A typical two-stage thermostat comprises two small mercury bulb contacts on a bi-metal sensor that close and open as a function of the movement of the bi-metal sensor in response to changing room temperatures. If the room temperature changes from a desired temperature set point, then the mercury bulb contacts close one at a time depending on how drastic of change in room temperature occurs. For example, the first mercury bulb will close with just a degree difference in temperature activating low heat, afterwards the second mercury bulb will close if the temperature difference increases another degree to activate high heat. Once the heating load is satisfied, both mercury bulb contacts will sequentially open or deactivate, thereby terminating the heating cycle. If a new heating load is requested, the furnace control repeats the same identical cycle. A disadvantage of furnace controls with two-stage thermostat is limited heating rates with large swings in room temperature. Single-stage thermostats are even more limiting since they only provide one heating rate creating even larger swings in room temperature.
Attempts have been made in designing thermostat algorithms that will provide multiple heating modes using a single-stage thermostat by the Carrier Corporation, the assignee of the present disclosure. Carrier disclosed in U.S. Pat. Nos. 5,340,028 and 5,337,952 methods for using a single-stage thermostat to control furnaces with multiple heating modes that adapt based on the previous cycle performance. However, a need for a thermostat algorithm for a modulating furnace still remains.